Many liquid media possess a relative degree of elasticity and tensile strength, and are classified as viscoelastic materials. Some liquid media are characterized by pronounced elasticity and high tensile strength, and exhibit unique characteristics during laminar flow, i.e., they are superelastic liquids.
One characteristic of superelastic liquids is a tendancy to recoil when deforming forces acting on them are removed, and the potential energy stored in the liquids as a result of deforming forces is converted into kinetic energy. Another unique characteristic of superelastic liquid media is the ability to build up resistance to deformation as shear stresses increase and to flow readily at shear stresses about certain values.
It has been found that these and other properties qualify such liquid media as particularly useful for controlling lost circulation in oil field drilling operations and in related applications.
The superelastic liquids which have the greatest utility are those prepared by crosslinking water-soluble polymers in aqueous solution. It has been found that polysaccharidic hydrocolloids can be crosslinked in aqueous solution to produce viscous liquid media which are highly elastic and have considerable tensile strength.
U.S. Pat. No. 2,644,762 describes a water-dispersible dry blend mixture comprising locust bean gum, sodium borate and glyoxal or pyruvic aldehyde. The mixture is dispersed in cold water, and the dispersion is heated above 200.degree. F. to produce a clear solution of high viscosity.
U.S. Pat. No. 2,968,581 describes a method of crosslinking polymeric polyhydroxy materials which involves reacting the said materials in the presence of water with a preformed polymeric acetal.
U.S. Pat. No. 2,976,274 describes a method for preparing a water-soluble thickening agent by reaction of gum arabic, dextran or corn fiber gum with a bifunctional crosslinking agent such as epihalohydrins or glycerol dihalohydrins.
U.S. Pat. No. 3,697,498 describes a novel crosslinking agent for use with aqueous solutions of polysaccharides to form enhanced shear rate thinning viscoelastic solutions, wherein the said crosslinking agent consists essentially of a chelating composition formed from an organic polyelectrolyte ligand component, and a compound of a metal of the first series of the transition elements of the Periodic Table.
U.S. Pat. No. 3,829,412 describes an improved procedure for modifying polysaccharides such as hemicellulose by crosslinking with a bifunctional etherifying reagent such as epichlorohydrin.
U.S. Pat. No. 3,836,465 describes a viscoelastic hydraulic fluid composition comprising an aqueous medium containing a mixture of polyalkylene oxide and lignosulfonate salt.
U.S. Pat. No. 4,183,765 describes a method of increasing the viscosity of a hydroxyalkyl cellulose solution by treatment with benzoquinone under controlled pH conditions.
Of general interest relative to the present invention, U.S. Pat. No. 4,287,078 describes novel defoaming agents which are prepared by the reaction of polyoxyalkylene polyols with polyglycidyl ethers.
The various non-Newtonian viscoelastic solutions disclosed in the prior art usually are characterized by one or more disadvantages which limit their applicability in important commercial operations, e.g., for use as hydraulic fluid vehicles in well-drilling and in secondary oil and gas recovery from subtetranean formations.
One disadvantage is that some high viscosity solutions are not sufficiently thixotropic in relation to low and high shear rates.
Another disadvantage is that with some methods of thickening aqueous solutions there is not sufficient control over the attainment and maintenance of the desired viscosity range and properties.
Another disadvantage is that with some methods of thickening aqueous solutions it is necessary as a subsequent procedure to add a pH control reagent to an aqueous solution treated with a thickening agent, to effect the desired rate of formation and degree of viscoelastic properties in the aqueous solution.
Another disadvantage of some viscoelastic solutions is an intolerance to the presence of inorganic electrolytes.
A further disadvantage of some viscoelastic solutions is a sensitivity to temperatures above about 60.degree. C., and a consequential degradation of rheological properties.
Accordingly, it is an object of this invention to provide a novel thickening agent adapted to impart thixotropic properties to an aqueous medium without a need for subsequent adjustment of pH conditions.
It is another object of this invention to provide a process for imparting viscoelastic properties to an aqueous medium.
It is a further object of this invention to provide a thixotropic aqueous solution which exhibits improved heat stability and tolerance to the presence of inorganic electrolytes.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.